The present invention relates to a ratio control for a continuously variable transmission.
U.S. Pat. No. 4,515,040 issued to Takeuchi et al. on May 7, 1985 discloses a ratio control system wherein a target engine speed that is determined based on various input signals, such as an accelerator pedal position, a vehicle speed and a shift lever position, is compared with an actual engine speed to give an engine speed error. A signal indicative of this engine speed error is fed to a shift control driver (as designated generally by the reference numeral 300 in FIG. 12 of the reference) which includes a differentiator and an integrator. Since the error indicative signal is used as inputs to the differentiator and integrator, the shift control driver produces an output that is proportional to a linear combination of the time rate-of-change of the error indicative signal and the time integral of the error indicative signal. In response to the output of the shift driver, a shift actuator effects a ratio control of a continuously variable transmission.
U.S. patent application Ser. No. 659,699 filed by Oshiage et al on Oct. 11, 1984, (now U.S. Pat. No. 4,729,103)describes a ratio control system for a continuously variable transmission wherein a target reduction ratio that is determined based on various input signals is compared with an actual reduction ratio to give a ratio error. A signal indicative of this ratio error is integrated with respect to time. The time integral of the ratio error indicative signal is multiplied with an integral gain, while the reduction ratio indicative signal is multiplied with a proportional gain. A shift actuator effects a ratio control in response to an output that is proportional to a linear combination of the time integral of the error indicative signal and the error indicative signal. In this ratio control system, if the integral gain is set to sufficiently large value for good response, an actual reduction ratio overshoots in the vicinity of a target reduction ratio to cause hunting phenomena to occur in the event a ratio error becomes large under acceleration, for example. If the integral gain is set small enough to suppress the above mentioned undesirable phenomena, the system response becomes poor.
In order to solve the above-mentioned problem, U.S. patent application Ser. No. 737,021 filed by Oshiage on May 22, 1985, (now U.S. Pat. No. 4,718,012) proposes a ratio control system for a continuously variable transmission wherein a ratio control is effected on a linear combination of the error and the time integral of the error, and the time integral is adjusted, to zero or a low level near zero when the absolute value of the error is greater than a predetermined value. Describing more specifically, when the absolute value of the error is greater than the predetermined value, the content of an integrator is cleared and the time integral action by the integrator is stopped so that a shift command signal drops immediately after the error has exceeded the predetermined value by a value that is proportional to the output of the integrator immediately before the error has exceeded the predetermined value. This measure is effective in restraining the overshoot and/or hunting phenomena. However, there are cases where the system response drops excessively. Another problem is that since the content of the integrator is cleared until the time integral by the integrator will be allowed to resume, the system response is not good after resumption of the time integral operation because the output of the integrator is reduced to zero or a low value.
An object of the present invention is to improve a ratio control system such that without any drop in system response, the occurrence of overshoot and hunting phenomena is prevented.